If you’re travelling in the UK and need medical advice, you may not be able to see a GP but will have access to things like the NHS 111 hotline for non-urgent medical advice.
If needing an in person appointment you can got to places locally to your address, like Urgent Care Centres (UCC) or other walk-in centres, to be seen by a physician.
You may even find it easier to see a private GP and claim it on your travel insurance, should you have it. There are many private GP doctor surgeries in London that will usually be able to see you same or next day. Expect a fee of about £100 though.